The present invention relates to a neutral point potential control method of a power conversion device, such as a three-phase neutral point clamp type inverter, for performing variable speed driving of a motor or system interconnection, and a power conversion device, such as an inverter servo drive, for performing variable speed driving of a motor, or a power conversion device for performing system interconnection.
Conventionally, as a neutral point potential control method of a three-phase neutral point clamp type inverter, there generally exists a method for carrying out control by applying a zero-phase voltage to an instruction voltage as disclosed in Japanese Patent No. 2821168 xe2x80x9cInverter device and AC motor driving systemxe2x80x9d, and a system in which a neutral point potential is controlled by adjusting a time of an output vector using a space voltage vector system.
FIG. 1 shows a basic structure of a three-phase neutral point clamp type inverter, and in the drawing, reference numeral 1 designates a three-phase AC power source; 2, a rectifying element; 3, 4, smoothing capacitors; 6 to 23, diodes; 24 to 35, IGBTs; and 36, a motor.
In FIG. 1, when a potential difference between a neutral point voltage (voltage of a connection point O of serial-connected smoothing capacitors of the inverter) and a negative bus voltage is Vcn, in the neutral point potential control, Vcn must be controlled to be a half voltage of a bus voltage Vpn of the inverter.
In a case where the three-phase neutral point clamp type inverter as shown in FIG. 1 selects an output vector shown in FIG. 2 and controls Vcn, as voltage vectors which can be used for control of Vcn, there are only twelve vector of xp(1), xn(1), xp(2), xn(2), xp(3), xn(3), yp(1), yn(1), yp(2), yn(2), yp(3), and yn(3). FIG. 3 shows connection states of a load and the smoothing capacitors of the inverter in the case where the twelve vectors are outputted.
When a load current flows in a direction of an arrow shown in FIG. 3, for example, in the connection state of xp(1) and xn(1) of a region 1, since directions of currents flowing to the neutral point become opposite to each other, if xp(1) and xn(1) are generated in such a minute time that a U-phase current does not change as shown in FIG. 4, Vcn rises at the time of the generation of xp(1), and drops at the time of the generation of xn(1), and if a generation time Tp of xp(1) is equal to a generation time Tn of xn(1), an average Vcn in a total generation time Tout (=Tp+Tn) of xp(1) and xn(1) becomes a constant voltage, and if Tp greater than Tn, then the average Vcn rises. If Tp less than Tn, the average Vcn drops.
It is understood that in this way, by adjusting the generation time ratio of the vectors arranged side by side in FIG. 3 (xp(1) and xn(1), yp(1) and yn(1), xp(2) and xn(2), yp(2) and yn(2), xp(3) and xn(3), yp(3) and yn(3)), Vcn can be controlled.
In the neutral point potential control in which the zero-phase voltage is applied to the instruction voltage, application of a positive zero-phase voltage is almost equivalent to lengthening of a generation time ratio of xp(1), xp(2), xp(3), yp(1), yp(2), or yp(3), and application of a negative zero-phase voltage is almost equivalent to lengthening of a generation time of xn(1), xn(2), xn(3), yn(1), yn(2), or yn(3).
In the system in which the space voltage vector is used, for example, when a certain voltage vector in a region i (i=1, 2, . . . , 6) is outputted, a total output time of xp(j) and xn(j) vectors is made Tx(i), a total output time of yp(k) and yn(k) vectors is made Ty(i), an output time of xp(j) is made Txp(j), an output time of xn(j) is made Txn(j), an output time of yp(k) is made Typ(k), and an output time of yn(k) is made Tyn(k), and when xcex1 is defined to set such a relation as
Txp(j)=xcex1Tx(i)
Txn(j)=(1xe2x88x92xcex1)Tx(i)
Typ(k)=xcex1Ty(i)
Tyn(k)=(1xe2x88x92xcex1)Ty(i)
(j=1 when i=1,j=2 when k=1 and i=2,j=2 when k=1 and i=3, j=3 when k=2 and i=4,j=3 when k=2 and i=5,j=1 when k=3 and i=6,k=3),
in an electrical driving state,
when xcex1 is made large, Vcn rises,
when xcex1 is made small, Vcn drops,
besides, in a regenerative state,
when xcex1 is made large, Vcn drops,
when xcex1 is made small, Vcn rises,
whereby, the neutral point potential can be controlled by adjusting xcex1.
However, in the conventional neutral point potential control method in which the zero-phase voltage is added to the instruction, since the neutral point potential control becomes impossible at a load power factor of approximately zero, as a method of performing the neutral point potential control without receiving the influence of the load power factor to solve this, there is a method as disclosed in Japanese Patent Unexamined Publication No. Hei. 9-182455 in which an even component of modulation instructions is superimposed as the zero-phase voltage, however, there has been a problem that an effect is not remarkable although the control is complicated.
Besides, as disclosed in Japanese Patent No. 2888104, although there is a method in which a corresponding neutral point potential period is adjusted according to a direction of a current of a predetermined phase, there has been a problem that in a multi-phase inverter, the control of inter-phase output voltage can not be carried out well.
Then, a first problem to be solved by the present invention is to provide a neutral point potential control method of a three-phase neutral point clamp type inverter in which without degrading the quality of an inter-phase output voltage, neutral point potential control can be carried out irrespective of a power factor by simple measurement or prediction of a phase current, and neutral point potential fluctuation due to load current unbalance at the time of load ground fault can also be suppressed, whereby the quality, stability and safety of the inverter can be improved.
Further, as a conventional PWM pulse generating method of a three-phase neutral point clamp type PWM inverter, as disclosed in Japanese Patent Unexamined Publication No. Hei. 5-146160, there is a unipolar modulation/dipolar modulation for outputting a pulse by comparing an amplitude instruction with a carrier wave, or as disclosed in Japanese Patent Unexamined Publication No. Hei. 5-292754, there is a system in which a generation time of each vector is calculated by using an idea of a space vector and a PWM pulse is generated. FIG. 5 is a vector diagram in which output voltage vectors of a three-phase neutral point clamp type inverter are shown on a plane. When a switch state in which a phase output terminal of the three-phase neutral point clamp type PWM inverter is connected to a positive bus is P, a switch state in which it is connected to a negative bus is N, and a switch state in which it is connected to a neutral line is O, and when they are arranged in the order of UVW of output phases, output voltage vectors which the three-phase neutral point clamp type inverter can take have 27 kinds of switch states as shown in FIG. 5.
Here, for convenience of explanation, the 27 kinds of switch states shown in FIG. 5, which the three-phase neutral point clamp type PWM inverter can take, are classified into groups of
zero vector
PPP: Op
OOO: Oo
NNN: On
x vector
POO, OPO, OOP: xp
ONN, NON, NNO: xn
y vector
PPO, OPP, POP: yp
OON, NOO, ONO: yn
z vector
PON, OPN, NPO, NOP, ONP: z
a vector
PNN, NPN, NNP: a
b vector
PPN, NPP, PNP: b
and division is made such that
regions surrounded by the zero vector, the x vector and the y vector are 1-1 to 6-1,
regions surrounded by the x vector, the a vector and the z vector are 1-2 to 6-2,
regions surrounded by the x vector, the y vector and the z vector are 1-3 to 6-3, and
regions surrounded by the y vector, the b vector and the z vector are 1-4 to 6-4.
In order that the three-phase neutral point clamp type PWM inverter outputs a certain voltage vector A in the regions shown in FIG. 5, vectors in a switch state nearest to the tip of the voltage vector are used, these vectors are successively generated, and a pulse width modulation (PWM) is performed so that a composite value of the vectors in a unit time becomes equal to the voltage vector A and the output voltage is obtained.
In the three-phase neutral point clamp type PWM inverter, generally, as shown in FIG. 6, in order to form a neutral point voltage, an even number of smoothing capacitors 3 and 4 are connected in series between a main circuit positive bus P and a negative bus N, and a neutral line is taken from a terminal 0 of a capacitor which has exactly a middle voltage between the positive bus P and the negative bus N and is used. In FIG. 6, reference numeral 1 designates a three-phase AC power source; 2, a rectifying diode bridge; 6 to 11, clamp diodes; 12 to 23, reflux diodes; 24 to 35, IGBTs; 36 to 38, current sensors; and 39, a load motor.
The neutral line 0 is connected as shown in FIGS. 7 and 8 according to the PWM inverter output load (the load motor 39) and a switch state of the PWM inverter. The potential of the neutral line (neutral point potential) is changed by a current for charging the capacitor from the positive bus/negative bus and a current from the connected load.
In the switch states shown in FIG. 7, a set of switch states in which although line-to-line voltages outputted to the load are the same, load phases connected to the neutral line are different (adjacent switch states in FIG. 7 are made a set), is used, and a time ratio of generation of the switch states of this set is adjusted, so that the neutral point potential can be finely controlled.
However, in the switch state shown in FIG. 8, the neutral point potential is changed by the phase current of the load connected to the neutral line and the time ratio of generation of this switch state, and there is no switch state to completely correct this, so that the neutral point potential fluctuation caused by the switch state of FIG. 8 must be corrected by using the switch state of FIG. 7.
Then, conventionally, as disclosed in Japanese Patent Unexamined Publication No. Hei. 2-261063, a zero-phase voltage is added to a percent modulation, and generation times of the switch states of the set shown in FIG. 7 are adjusted, so that the neutral point potential fluctuation is controlled without changing the line-to-line output voltage supplied to the load. Besides, also in a space vector method, a voltage vector to be outputted is outputted so as to use the set of the switch states shown in FIG. 7, and the generation times of the switch states of the set are adjusted so that the neutral point potential is controlled.
In the three-phase neutral point clamp type inverter, the neutral point potential fluctuation at the time of the switch state of FIG. 8 is determined by the phase current of the load connected to the neutral line from FIG. 8. When the phase current (when load power factor=1) of the load connected to the neutral line is depicted in FIG. 9, since the direction of a phase current 110 of the load connected to the neutral line is always inverted before the output voltage vector is changed by 120 degrees, there has been a problem that the neutral point potential fluctuates at a frequency three times as high as the output frequency by this influence.
Further, in the case where the percent modulation is rather high, the generation time ratio of the switch state shown in FIG. 8 becomes higher than that of the switch state shown in FIG. 7, and in an excessive modulation state in which the percent modulation exceeds about 1.15, there has been a problem that the generation time ratio of the switch state shown in FIG. 7 becomes completely zero, and the fluctuation of the neutral point potential caused by the switch state shown in FIG. 8 can not be suppressed.
Then, a second problem to be solved by the present invention is to suppress the neutral point potential fluctuation of the three-phase neutral point clamp type inverter, to enable adjustment of the neutral point potential at the time of excessive modulation, which has been conventionally impossible, and to realize the improvement of safety and the improvement of output voltage quality.
In order to solve the first problem, a neutral point potential control method of a neutral point clamp type inverter of the present invention is
(1) a neutral point potential control method characterized in that
when a state in which a phase output terminal of a three-phase neutral point clamp type inverter is connected to a positive bus voltage point of the inverter is P, a state in which it is connected to a neutral point of a bus of the inverter is O, and a state in which it is connected to a negative bus voltage point of the inverter is N, and
when an output voltage is expressed as a space vector such that as a three-phase output state of the inverter in order of a U phase, a V phase and a W phase,
an output state which becomes POO is a vector xp(1),
an output state which becomes ONN is a vector xn(1),
an output state which becomes PPO is a vector yp(1),
an output state which becomes OON is a vector yn(1),
an output state which becomes OPO is a vector xp(2),
an output state which becomes NON is a vector xn(2),
an output state which becomes OPP is a vector yp(2),
an output state which becomes NOO is a vector yn(2),
an output state which becomes OOP is a vector xp(3),
an output state which becomes NNO is a vector xn(3),
an output state which becomes POP is a vector yp(3), and
an output state which becomes ONO is a vector yn(3),
in the three-phase neutral point clamp type inverter,
in a case where an angle of a voltage vector to be outputted by the inverter is contained by the vector yp(3) and the vector yp(1), a generation time ratio of the vector xp(1) to the vector xn(1) is changed according to a direction of a current of the U phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(1) and the vector yp(2), a generation time ratio of the vector xp(2) to the vector xn(2) is changed according to a direction of a current of the V phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(2) and the vector yp(3), a generation time ratio of the vector xp(3) to the vector xn(3) is changed according to a direction of a current of the W phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(1) and the vector xp(2), a generation time ratio of the vector yp(1) to the vector yn(1) is changed according to the direction of the current of the W phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(2) and the vector xp(3), a generation time ratio of the vector yp(2) to the vector yn(2) is changed according to the direction of the current of the U phase, and
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(3) and the vector xp(1), a generation time ratio of the vector yp(3) to the vector yn(3) is changed according to the direction of the current of the V phase,
whereby a neutral point voltage of the three-phase neutral point clamp type inverter is stabilized.
(2) A neutral point potential control method is characterized in that
when a state in which a phase output terminal of a three-phase neutral point clamp type inverter is connected to a positive bus voltage point of the inverter is P, a state in which it is connected to a neutral point of a bus of the inverter is O, and a state in which it is connected to a negative bus voltage point of the inverter is N, and
when an output voltage is expressed as a space vector such that as a three-phase output state of the inverter in order of a U phase, a V phase and a W phase,
an output state which becomes POO is a vector xp(1),
an output state which becomes ONN is a vector xn(1),
an output state which becomes PPO is a vector yp(1),
an output state which becomes OON is a vector yn(1),
an output state which becomes OPO is a vector xp(2),
an output state which becomes NON is a vector xn(2),
an output state which becomes OPP is a vector yp(2),
an output state which becomes NOO is a vector yn(2),
an output state which becomes OOP is a vector xp(3),
an output state which becomes NNO is a vector xn(3),
an output state which becomes POP is a vector yp(3), and
an output state which becomes ONO is a vector yn(3),
in the three-phase neutral point clamp type inverter,
in a case where an angle of a voltage vector to be outputted by the inverter is contained by the vector yp(3) and the vector yp(1), a generation time ratio of the vector xp(1) to the vector xn(1) is changed according to a direction of a sum of currents of the V phase and the W phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(1) and the vector yp(2), a generation time ratio of the vector xp(2) to the vector xn(2) is changed according to a direction of a sum of currents of the U phase and the W phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(2) and the vector yp(3), a generation time ratio of the vector xp(3) and the vector xn(3) is changed according to a direction of a sum of currents of the U phase and the V phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(1) and the vector xp(2), a generation time ratio of the vector yp(1) to the vector yn(1) is changed according to the direction of the sum of the currents of the U phase and the V phase,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(2) and the vector xp(3), a generation time ratio of the vector yp(2) to the vector yn(2) is changed according to the direction of the sum of the currents of the V phase and the W phase, and
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(3) and the vector xp(1), a generation time ratio of the vector yp(3) to the vector yn(3) is changed according to the direction of the sum of the currents of the U phase and the W phase,
whereby a neutral point voltage of the three-phase neutral point clamp type inverter is stabilized.
(3) A neutral point potential control method is characterized in that
when a state in which a phase output terminal of a three-phase neutral point clamp type inverter is connected to a positive bus voltage point of the inverter is P, a state in which it is connected to a neutral point of a bus of the inverter is O, and a state in which it is connected to a negative bus voltage point of the inverter is N, and
when an output voltage is expressed as a space vector such that as a three-phase output state of the inverter in order of a U phase, a V phase and a W phase,
an output state which becomes POO is a vector xp(1),
an output state which becomes ONN is a vector xn(1),
an output state which becomes PPO is a vector yp(1),
an output state which becomes OON is a vector yp(1),
an output state which becomes OPO is a vector xp(2),
an output state which becomes NON is a vector xn(2),
an output state which becomes OPP is a vector yp(2),
an output state which becomes NOO is a vector yp(2),
an output state which becomes OOP is a vector xp(3),
an output state which becomes NNO is a vector xn(3),
an output state which becomes POP is a vector yp(3), and
an output state which becomes ONO is a vector yn(3),
in the three-phase neutral point clamp type inverter,
in a case where an angle of a voltage vector to be outputted by the inverter is contained by the vector yp(3) and the vector yp(1), a current Iu of the U phase is compared with a sum of a current Iv of the V phase and a current Iw of the W phase, and if Iu and Iv+Iw have a same sign and
if |Iu | less than |Iv+Iw |, then generation of the vector xp(1) is suppressed,
if |Iu | greater than |Iv+Iw |, then generation of the vector xn(1) is suppressed,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(1) and the vector yp(2), the current Iv of the V phase is compared with a sum of the current Iu of the U phase and the current Iw of the W phase, and if Iv and Iu+Iw have a same sign and
if |Iv| less than |Iu+Iw|, then generation of the vector xp(2) is suppressed,
if |Iv| greater than |Iu+Iw|, then generation of the vector xn(2) is suppressed,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector yp(2) and the vector yp(3), the current Iw of the W phase is compared with a sum of the current Iu of the U phase and the current Iv of the V phase, and if Iw and Iu+Iv have a same sign and
if |Iw| less than |Iu+Iv|, then generation of the vector xp(3) is suppressed,
if |Iw| greater than |Iu+Iv|, then generation of the vector xn(3) is suppressed,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(1) and the vector xp(2), the current Iw of the W phase is compared with the sum of the current Iu of the U phase and the current Iv of the V phase, and if Iw and Iu+Iv have a same sign and
if |Iw| less than |Iu+Iv|, then generation of the vector yn(1) is suppressed,
if |Iw| greater than |Iu+Iv|, then generation of the vector yp(1) is suppressed,
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(2) and the vector xp(3), the current Iu of the U phase is compared with the sum of the current Iv of the V phase and the current Iw of the W phase, and if Iu and Iv+Iw have a same sign and
if |Iu| less than |Iv+Iw|, then generation of the vector yn(2) is suppressed,
if |Iu| greater than |Iv+Iw|, then generation of the vector yp(2) is suppressed, and
in a case where the angle of the voltage vector to be outputted by the inverter is contained by the vector xp(3) and the vector xp(1), the current Iv of the V phase is compared with the sum of the current Iu of the U phase and the current Iw of the W phase, and if Iv and Iu+Iw have a same sign and
if |Iv| less than |Iu+Iw|, then generation of the vector yn(3) is suppressed,
if |Iv| greater than |Iu+Iw|, then generation of the vector yp(3) is suppressed,
whereby a neutral point voltage of the three-phase neutral point clamp type inverter is stabilized.
According to the neutral point potential control method of the three-phase neutral point clamp type inverter as described in (1) and (2), the neutral point potential control becomes possible irrespective of the load power factor, and the control becomes simple since the direction of the load current is merely detected. Besides, according to the neutral point potential control of the three-phase neutral point clamp type inverter as described in (3), it is possible to suppress an abrupt neutral point potential fluctuation in a case where the inverter output is grounded, and the stability is improved.
Besides in order to solve the second problem, a three-phase neutral point clamp type PWM inverter device is
(1) a three-phase neutral point clamp type PWM inverter device including neutral point clamp type PWM inverters for three phases, each of which includes a positive bus, a negative bus, and a neutral line, and in each of which a first and a second switch elements and a third and a fourth switch elements are connected in series between the positive bus and a phase voltage output terminal and between the negative bus and the phase output terminal, respectively, and a connection point of the first and the second switch elements and a connection point of the third and the fourth switch elements are connected to the neutral line through clump elements, respectively, and characterized in that a time of a three-phase output voltage of six switch states in which the positive bus, the negative bus, and the neutral line are respectively connected to the three-phase phase output terminals is suppressed to be a first set value or less, and an insufficiency of the output voltage resulting from suppression to the first set value or less is compensated by six switch states among eight switch states in which the three-phase phase output terminals are respectively connected to the positive bus or the negative bus except two switch states in which all of the three-phase phase output terminals are connected to the positive bus or the negative bus at the same time.
(2) In the three-phase neutral point clamp type PWM inverter device of (1), the three-phase neutral point clamp type PWM inverter device is characterized in that when the six switch states in which the suppression to the first set value or less is carried out are transferred to the six switch states for compensating the insufficiency of the output voltage, the switch state of only one phase of the neutral point clamp type PWM inverter is changed.
(3) In the three-phase neutral point clamp type PWM inverter device of (1) or (2), the three-phase neutral point clamp type PWM inverter device is characterized in that the first set value is changed according to a value of a percent modulation index.
(4) In the three-phase neutral point clamp type PWM inverter device of (1) or (2), the three-phase neutral point clamp type PWM inverter device is characterized in that the first set value is changed according to a direction of a current flowing to the neutral line or a phase of an output current.
(5) In the three-phase neutral point clamp type PWM inverter device of (1) or (2), the three-phase neutral point clamp type PWM inverter device is characterized in that the first set value is changed according to a voltage value of the neutral line.